Nonlinear optical crystal such as lithium niobate or lithium tantalate single crystal has a high secondary nonlinear optical constant. When a periodic domain polarization inversion structure is formed in the above crystals, a second-harmonic-generation (SHG) device of a quasi-phase-matched (QPM) system can be realized. Further, when a waveguide is formed within this periodic domain inversion structure, the high-efficiency SHG device can be realized and further, applied to optical communication, medical science, photochemistry, various optical measurements over a wide range.
Conventionally, it is required a light source of fundamental wave called exiting light from the outside of an SHG device. For example, in the case of an SHG laser oscillating green light, as the light source of the fundamental wave, it may be used a solid laser oscillating light having a wavelength of 1064 nm obtained by combining a semiconductor laser of 800 nm band with a laser crystal such as yttrium vanadate (YVO4) for example. Such crystal material for laser oscillation includes, in addition to YVO4, yttrium aluminum garnet (YAG), gadolinium vanadate (GdVO4), potassium gadolinium tungstate (KGW), potassium yttrium tungstate (KYW) or the like.
For improving the efficiency of the SHG laser, it is effective to process the laser crystal into an waveguide in addition to the SHG device.
However, as the crystal for the SHG device and laser crystal are materials hard to be processed, it is difficult to form the optical waveguide having a low light propagation loss by the processing. For example, in the case of mechanical processing, the materials are susceptible to tipping, resulting in a cause of propagation loss. Further, in the case of laser ablation, the improvement of the productivity is limited, and further, tipping occurs at corners of the optical waveguide, and side faces and processed bottom face are roughened, as the processing rate is improved. The propagation property of the waveguide is thereby deteriorated.
Further, in the case that a wavelength converting device and optical modulation device is designed, it may be required to make the route of the optical waveguide curved, warped or branched. However, according to the mechanical processing or laser ablation, it is difficult to move a stage for processing in a direction perpendicular to the travelling direction. Further, even if the stage for the processing is travelled in the crossing direction, different from the case that the processing is performed as the stage is travelled straightforwardly, it is proved that the cross sectional shape of the ridge groove is deformed to result in an increase of the propagation loss.
On the other hand, according to Patent document 1, it is disclosed that stripes of a dielectric material are loaded on a thin layer of potassium niobate by lift-off process to form a channel and loaded type optical waveguide. According to such loaded type optical waveguide, it is possible to make the travelling direction of the optical waveguide curved and branched.